Cell formation control method, a mobile communications system, and a base station and a mobile station used therein

ABSTRACT

When a base station of a mobile communications system attempts to reduce its cell radius for power saving, frequency re-use enhancement and the like, a risk is that an ongoing communication between a mobile station served by the base station may be disconnected, which is avoided by: the base station transmitting information about a new cell radius to mobile stations in the current cell; the mobile stations responding to the base station with a positive or a negative reply determined from the information received from the base station; the base station suspending or revising the attempted cell radius reduction, according to the responses from the mobile stations; and further by the mobile stations being capable of performing hand-over more efficiently than ever owing to abilities to detect final cell radii of adjacent base stations.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a control method ofcell formation and a mobile communications system therewith, andspecifically relates to the cell formation method and the mobilecommunications system therewith, wherein one or more mobile stations areserved by a base station that is capable of changing and securing aservice area of the mobile stations by changing the cell formation whilecommunications are going on.

[0003] The present invention further relates to the base station andmobile station that can communicate according to the cell formationcontrol method.

[0004] 2. Description of the Related Art

[0005] An area where a base station can provide a communication service,i.e., a cell, depends on the quality of a control signal received by amobile station, the control signal being for connecting the mobilestation with the base station. When the base station serves a cell, aconventional method has been that transmission electric power of thecontrol signal is predetermined, and the predetermined value is used. Inrecent years, an autonomous cell formation control method has beenproposed, wherein a base station checks cell formation of adjacentcells, and adjusts its cell formation such that areas not covered by theadjacent cells are efficiently covered. The autonomous cell formationcontrol method mediates traffic congestion by controlling the cellformation according to a congestion state of the adjacent cells, and canraise frequency use efficiency. If the technology of such autonomouscell formation control is applied to a mobile communications system, abase station will be capable of changing cell formation, i.e.,increasing and decreasing the radius of the cell, while communicationsare continuing.

[0006] When the cell radius is expanded, an increased number of basestations will become available to a mobile station that is incommunication with a first base station. The communication connection tothe first base station can be switched to a second base station, if thesecond base station is carrying a lower amount of traffic, such that ahigher through put may be obtained.

[0007] On the other hand, when the cell radius of the first base stationwith which the mobile station is connected is reduced, it is necessaryfor the mobile station to switch the ongoing communication connection toanother base station before the service of the first base station to themobile station becomes unavailable.

[0008] However, according to the autonomous cell formation control of aconventional base station, a relief is not provided to the mobilestation that would be made outside the service area of the base stationbefore the base station actually reduces its service area.

[0009] For example, as shown in FIG. 16, when a base station 3 reducesits service area from an area indicated by a dotted line to an areaindicated by a solid line, although a mobile station 3 served by thebase station 3 is thereby put outside the reduced service area of thebase station 3, the mobile station 3 can be switched to the base station2 by hand-over, such that the communication can continue. However, whena base station 1 reduces its service area from an area indicated by adotted line to an area indicated by a solid line, there is no adjacentbase station to perform hand-over for a mobile station 1 that is beingserved by the base station 1. Consequently, when the base station 1reduces its service area, the mobile station 1 will be put outside thereduced service area, and the communication will be disconnected.

[0010] Thus, the conventional problem is that a normal communication ofa mobile station with the first base station cannot be continued due toa disconnection, a packet loss and the like, when the first base stationreduces its cell radius, i.e., service area.

[0011] Even if there is the second base station available for theservice to be continued in an adjacent area, hand-over has to beperformed from the first base station to the second base station.Further, there is a possibility that the second base station may laterreduce its service area. In this case, further hand-over is requiredfrom the second base station to a third base station, or back to thefirst base station, as the case may be.

SUMMARY OF THE INVENTION

[0012] It is a general object of the present invention to provide amethod, a system using the method, and a base station and a mobilestation that operate in the system, which substantially obviate one ormore of the problems caused by the limitations and disadvantages of therelated art.

[0013] Features and advantages of the present invention will be setforth in the description that follows, and in part will become apparentfrom the description and the accompanying drawings, or may be learned bypractice of the invention according to the teachings provided in thedescription. Objects as well as other features and advantages of thepresent invention will be realized and attained by the method, thesystem using the method, and the base station and the mobile stationthat operate in the system, which are particularly pointed out in thespecification in such full, clear, concise, and exact terms as to enablea person having ordinary skill in the art to practice the invention.

[0014] To achieve these and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, theinvention provides the method, the system, the base station and themobile station that enable the system to function as summarized below.

[0015] The base station, serving a cell, is equipped with a facility tochange formation of the cell, typically, the radius of the cell withinwhich the service of the base station is available, and a facility toprovide information about a contemplated cell radius change to one ormore mobile stations in the cell. Each mobile station is capable oftransmitting a request not to change the cell radius, and also capableof searching for another base station to which a hand-over is to takeplace, if the mobile station determines that the mobile station will beplaced outside the cell if the cell radius change is actually performedby the base station. Then, the base station, upon receiving the request,is capable of suspending the contemplated cell radius change.

[0016] An extension to the above is that the mobile station is capableof calculating the distance between the base station and the mobilestation, using signal strength of a signal from the base station, andother information, and the mobile station transmits information aboutthe distance to the base station, such that the base station can adjustthe cell radius enough to cover the mobile station concerned.

[0017] As an extension to the above, the base station is equipped with afacility to change the formation of the cell, i.e., the cell radius,gradually in stages until the contemplated cell radius change is finallyreached, and a facility to inform the mobile stations about a next stagecell radius one by one at a predetermined interval. The mobile stationis capable of transmitting the request not to change the cell radiuswhen a cell radius change of any of the stages that is contemplated willplace the mobile station outside the cell. This contributes to reducingprocessing load of the cell, because the number of control signals fromthe mobile stations to the base station in a given period of time isreduced, with transmission of the control signals being distributed overthe stages.

[0018] A further extension to the above is that the base station iscapable of transmitting information both about the next stage cellradius and about the cell radius in the final stage. In this case, themobile station is informed of the final cell radius from the beginning,and is enabled to attempt a hand-over to a second base station if thefinal cell radius will not cover the mobile station, even if a cellradius of any prior stage may cover the mobile station. This alleviatesfrequency and traffic of the hand-over. Further, in this case, even ifthe second base station is in the process of a cell radius change, themobile station can select a base station that will provide the mostfavorable communication conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a figure showing an example of a structure of a mobilecommunications system to which a cell formation control method of anembodiment of the present invention is applied;

[0020]FIG. 2 is a figure showing a structure of a base station of thepresent invention;

[0021]FIG. 3 is a figure showing a structure of a mobile station of thepresent invention;

[0022]FIG. 4 is a figure showing an outline of a first embodiment of acell radius changing process;

[0023]FIG. 5 is a flowchart showing steps of a process performed by abase station of the first embodiment;

[0024]FIG. 6 is a flowchart showing steps of a process performed by amobile station of the first embodiment;

[0025]FIG. 7 is a figure showing an outline of a second embodiment ofthe cell radius change process;

[0026]FIG. 8 is a flowchart showing steps of a process performed by abase station of the second embodiment;

[0027]FIG. 9 is a flowchart showing a continuation of the steps of theprocess performed by the base station of the second embodiment(continuation of FIG. 8);

[0028]FIG. 10 is a flowchart showing steps of a process performed by amobile station of the second embodiment;

[0029]FIG. 11 is a figure showing an outline of a third embodiment ofthe cell radius change process;

[0030]FIG. 12 is a flowchart showing steps of a process performed by thebase station of the third embodiment;

[0031]FIG. 13 is a flowchart showing a continuation of the steps of theprocess performed by the base station of the third embodiment(continuation of FIG. 12);

[0032]FIG. 14 is a flowchart showing steps of a process performed by themobile station of the third embodiment;

[0033]FIG. 15 is a flowchart showing a continuation of the steps of theprocess performed by the mobile station of the third embodiment(continuation of FIG. 14); and

[0034]FIG. 16 is a figure explaining a conventional technology.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Hereafter, embodiments of the present invention are explainedbased on attached figures.

[0036] A mobile communications system to which the cell formationcontrol methods of the embodiments of the present invention are appliedis structured as shown in FIG. 1.

[0037] In FIG. 1, the mobile communications system is, for example, aPDC (Personal Digital Cellular) system, and includes a base station andtwo or more mobile stations. In this example, a mobile station A 100 iscommunicating with a base station A 200, and a mobile station B 110 anda mobile station C 120 are communicating with a base station B 210, andeach base station is capable of changing cell formation. The cellformation is changed according to a cell formation change of an adjacentbase station, a congestion situation, and the like. For example, thebase station A 200 is capable of changing the cell formation from A1 toA2. The base station B 210 is also capable of changing its cellformation. A change of the cell formation can be realized by changingtransmission electric power of a perch channel (a control channel usedin order that a mobile station may synchronize with a base station),thereby a cell radius, i.e., the radius of a service area, is changed.Therefore, a change of transmission electric power is equivalent to acell formation change. Further, the base station may use a sectorantenna. In that case, a change of the transmission electric power ofany of one, more than one, and all sectors is regarded as a cellformation change.

[0038] In the present invention, when the base station A 200, forexample, carries out a cell formation change, i.e., a service area(=cell) change, e.g., from A1 to A2, it is carried out such that themobile station A 100 does not come outside of the service area.According to the present invention, the base station performs the cellformation change based on a response from the mobile station thatinforms the base station whether or not the mobile station will beplaced outside of the service area, thereby the base station can adjustthe cell formation change such that the mobile station can remain insidethe service area. Consequently, disconnection of communication of themobile station, due to the cell formation change, can be prevented.

[0039] The base station is structured as shown in FIG. 2, for example.

[0040]FIG. 2 is a block diagram of the base station of the presentinvention.

[0041] The base station includes a receiving antenna 11, a receivingunit 12, a control signal detection unit 13, a control informationprocessing unit 14, a cell formation processing unit 15, an informationsignal processing unit 16, a transmitting unit 17, and a transmittingantenna 18. In FIG. 2, an arrow shows a flow of a signal.

[0042] The receiving unit 12 receives a signal transmitted from themobile station through the receiving antenna 11, and provides thereceived signal to the control signal detection unit 13 of the followingstage. In the control signal detection unit 13, a change stop signalthat is the control signal transmitted by the mobile station is detectedfrom the signal received by the receiving unit 12, and informationcontained in the signal is provided to the control informationprocessing unit 14 of the following stage. The control informationprocessing unit 14 extracts and temporarily stores the informationprovided by the control signal detection unit 13. Here, since the changestop signal may be transmitted from two or more mobile stations, theinformation is stored with a mobile station identifier corresponding toeach mobile station. Then, the control information processing unit 14provides the cell formation processing unit 15 with the informationincluding a value of the cell radius (L_(MAX)) that is to be used in thecell formation change. Details about L_(MAX) will be described later.The cell formation processing unit 15 provides the information signalprocessing unit 16 with a new cell radius R1 that the cell formationchange is to take. Further, the information signal processing unit 16provides the new cell radius R₁ (changed cell radius) to thetransmitting unit 17. However, at this juncture, in the case thatL_(MAX) is provided by the control information processing unit 14, R₁,is set with the value of L_(MAX), and then, R1 is provided to thetransmitting unit 17 through the information signal processing unit 16.The information signal processing unit 16 updates contents of aninformation signal, and provides it to the transmitting unit 17. Thetransmitting unit 17 transmits the updated information signal to all themobile stations in the cell. Further, the transmitting unit 17 isequipped with a function, such as transmission power control, to changethe cell radius to R₁.

[0043] On the other hand, if no change stop signals from the mobilestations are detected by the control signal detection unit 13, the cellformation processing unit 15 provides R1, with no regard to L_(MAX), tothe transmitting unit 17, and cell formation change is carried out inthe transmitting unit 17.

[0044] The mobile station is structured as shown in FIG. 3, for example.

[0045]FIG. 3 is a block diagram of the mobile station of the presentinvention.

[0046] The mobile station includes a receiving antenna 21, a receivingunit 22, a connectable base station detection unit 23, a distancecalculation unit 24, an information signal detection unit 25, a cellradius comparison unit 26, a hand-over processing unit 27, a controlsignal processing unit 28, a transmitting unit 29, and a transmittingantenna 30. In FIG. 3, an arrow shows a flow of a signal.

[0047] The receiving unit 22 receives a signal transmitted from the basestation through the receiving antenna 21, and an information signalcontained in the received signal is provided to the information signaldetection unit 25 and the connectable base station detection unit 23. Inthe connectable base station detection unit 23, information required todetermine a hand-over destination, such as the number of base stationsthat can communicate with the mobile station, a base station identifier,a receiving level, and the like, is extracted from the informationsignal provided by the receiving unit 22, and the information isprovided to the hand-over processing unit 27. The information in thehand-over processing unit 27 contains information about hand-overdestination base station candidates when the cell radius comparison unit26 requires a hand-over control.

[0048] In the information signal detection unit 25, the informationabout the cell radius is extracted from the information signal receivedfrom the receiving unit 22. The information here includes, for example,the received electric power P_(r) of the signal, the new cell radius R₁if the cell formation change actually takes place, and the present basestation transmission electric power P_(s), and the like. The value ofthe cell radius after change R₁ is provided to the cell radiuscomparison unit 26, and other values, such as a value of P_(s), whichare needed for calculating a distance (distance between the base stationand the mobile station) are provided to the distance calculation unit24.

[0049] The cell radius comparison unit 26 checks whether or not themobile station will remain within the service area of the base stationby comparing the new cell radius after change R₁ with the distancebetween the mobile station and the base station. If it is determinedthat the mobile station will become outside the service area if the cellradius is actually changed, a hand-over direction is provided to thehand-over processing unit 27 or a change stop direction is provided tothe control signal processing unit 28, depending on whether another basestation is available for hand-over. The control signal processing unit28 generates either of the change stop signal or the control signalrelative to hand-over, and the generated signal is transmitted from thetransmitting unit 29.

[0050] Next, an outline of the first embodiment of cell radius changeprocessing of the present invention is described with reference to FIG.4.

[0051] Section (a) of FIG. 4 shows a mobile communications systemreferenced to explain the outline of the first embodiment. In thisfigure, mobile stations A and B are present in the service area(R₀=initial cell radius) that a base station forms, and each mobilestation can communicate with the base station. The base station is tochange the cell radius from the initial radius R₀ to a new cell radiusR₁.

[0052] In FIG. 4, the distance between the base station and the mobilestation A is expressed as L_(A), and the distance between the basestation and the mobile station B is expressed as L_(B).

[0053] Section (b) of FIG. 4 shows an outline of the cell radius changeprocessing that the base station performs when only one mobile station Ais present in the cell radius R₀ of the base station in the mobilecommunications system of the section (a) of FIG. 4. Here, the horizontalaxis of this graph expresses the time lapse starting when the basestation proposes a new cell radius, and the vertical axis expresses thedistance (cell radius) from the base station concerned.

[0054] In the section (b) of FIG. 4, at the time of t=0, the basestation starts proposing to change the cell formation and providesinformation about the change to all the mobile stations in the cell as apart of the information signal. During a period between t=0 and t=T, theinformation signal is transmitted to the mobile stations, and the cellradius is made small at t=T. The cell radius after change is expressedby R₁. Here, it is assumed that the position of the mobile station A isnot changed during the period. As the section (a) of FIG. 4 shows, themobile station A is within the service area before and after the cellradius change. In this case, no special processing is necessary as tothe mobile station A.

[0055] Section (c) of FIG. 4 shows an outline of the cell radius changeprocessing where only the mobile station B is present in the cell radiusR₀ formed by the base station in the mobile communications system asshown by the section (a) of FIG. 4.

[0056] When the cell radius is made small as shown in the section (b) ofFIG. 4, the mobile station B becomes outside of the service area aftert=T at which time the cell radius is made small. However, in the presentinvention, the mobile station B is warned in advance of the cell radiuschange, that is, the mobile station B becoming outside the service areaif the cell radius is actually made small according to the informationsignal received during the period between t=0 and t=T. Consequently, themobile station B can attempt to hand-over the existing communicationconnection to another base station. The hand-over is performed ifanother base station is available, avoiding a disconnection. If theother base station is not available, the mobile station B transmits achange stop signal to the base station, such that the base stationsuspends the cell radius change, or revises the new cell radius suchthat the mobile station B can continuously be served.

[0057] Next, details of the processing procedure of the first embodimentare explained.

[0058]FIG. 5 is a flowchart that shows the processing procedure of thebase station, and FIG. 6 is a flowchart that shows the processingprocedure of the mobile station.

[0059] First, the processing procedure of the base station is explained,referring to FIG. 5.

[0060] When the base station decides to make a cell radius change, theinformation signal processing unit 16 generates an information signalthat includes information contents such as the new cell radius R₁ andthe present transmission electric power P_(s). The information signal isperiodically and repeatedly transmitted (S1) to the mobile stations inthe cell, a timer is started, and a response from the mobile station(S2) is waited for. The time length from the transmission start time ofthe information signal to scheduled time of the cell radius changeimplementation is set at T as shown in FIG. 4. When the predeterminedtime elapses, i.e., timeout, (YES at S2), the control signal detectionunit 13 checks whether a change stop signal has been received from oneor more of the mobile stations (S3). If the change stop signal has beenreceived from one or more mobile stations (YES at S3), a value L_(i)that indicates the distance between a mobile station and the basestation is extracted from the change stop signal, and the value isprovided to the control information processing unit 14 (S4).

[0061] Otherwise, that is, if it is determined that no change stopsignals have been received from the mobile stations (NO at S3), theprocess ends, and returns to the first processing step.

[0062] The control information processing unit 14 determines the largestvalue L_(MAX) of the distance values L_(i) and the largest value L_(MAX)is provided to the cell formation processing unit 15 (S5). The cellformation processing unit 15 sets the value L_(MAX) as the value of thenew cell radius R₁ such that the mobile station that gives the largestvalue can remain within the service area of the base station (S6). Then,the process ends, and returns to the first processing step, and repeatsthe above-mentioned procedure.

[0063] Next, the processing procedure of the mobile station is describedwith reference to FIG. 6.

[0064] When the receiving unit 22 of the mobile station receives theinformation signal from the base station with which it is communicating,the received signal is provided to the information signal detection unit25. The information signal detection unit 25 extracts data such as thereceived electric power Pr, the new cell radius R₁ after change, and thepresent transmission electric power P_(S) of the base station from thereceived signal (S11), and provides the data to the distance calculationunit 24. The distance calculation unit 24 calculates the distance L_(i)between the mobile station and the base station concerned, applying thereceived electric power P_(r) and P_(S) received from the informationsignal detection unit 25 to a formula (Formula 1) that gives arelationship between the signal electric power and the distance (S12).

P _(r) =A×P _(S)×(1/L _(i))⁴,  (Formula 1)

[0065] where

[0066] A is a coefficient indicative of the propagation path situationbetween the base station and the mobile station.

[0067] Although the present embodiment employs Formula 1, for example,for calculating the distance between the mobile station and the basestation, another formula of the relationship between the signal electricpower and the distance suitable for the propagation path characteristicsof the system can be used, and the present invention is not limited tousing Formula 1.

[0068] When the distance calculation unit 24 calculates L_(i), thecalculation result is provided to the cell radius comparison unit 26.The cell radius comparison unit 26 compares L_(i) with R₁ (S13). If itis determined that R₁, is larger than L_(i) (NO at S13) the process endsbecause it means that the mobile station will remain within the servicearea after the cell radius change. Otherwise, if it is determined thatR₁ is smaller than L₁, (YES at S13) the process moves to the next step(S14) where it is determined whether a hand-over to another base stationis possible (S14). The determination (S14) is performed as follows.

[0069] The hand-over processing unit 27 receives information items sentone by one from the connectable base station detection unit 23, anddetermines the availability of a hand-over. The result of thedetermination (availability of base stations for hand-over) isperiodically provided to the cell radius comparison unit 26. The cellradius comparison unit 26 determines whether a hand-over to another basestation is possible using the information about the availability ofconnectable base stations provided by the hand-over processing unit 27.

[0070] When the cell radius comparison unit 26 performs determining asmentioned above (S14) and determines that a hand-over to another basestation is possible (NO at S14), it directs the control signalprocessing unit 28 to generate a control signal that is needed by themobile station for carrying out hand-over. Then, the control signal isprovided to the mobile station such that the mobile station carries outthe hand-over to another base station, and continues communication.

[0071] Otherwise, that is, when it is determined that the hand-over toanother base station is impossible (YES at S14), the transmitting unit29 of the mobile station concerned transmits the change stop signal thatcontains L_(i) to the base station (S16). According to this embodiment,a mobile station is capable of preventing the base station from reducingthe cell radius any smaller than L_(i) by transmitting information thatcontains the value of L_(i) to the base station.

[0072] Next, an outline of the second embodiment of the cell radiuschange processing is described with reference to FIG. 7, wherein a basestation changes cell formation based on the cell formation controlmethod of the present invention.

[0073] In FIG. 7, the horizontal axis expresses the time lapse from thetime when the base station decides to change the cell radius, and thevertical axis expresses the distance (cell radius) from the basestation, like FIG. 4. Here, in the base station, R₀ represents thepresent cell radius, and R₁ represents the new cell radius after thechange that the base station proposes to perform. In the secondembodiment of the present invention, the cell radius change from R₀ toR₁ is performed in N steps, without changing all at once. For example,the cell radius change is carried out in three steps (N=3) in thisembodiment, namely, the cell radius is to be reduced from R₀ toR_(t)=R₀−R_(L), R_(t)=R₀−2R_(L), and then to R_(t)=R₀−3R_(L)=R₁, asshown in this graph. Specifically, the following R_(t) values of stagechange cell radius are transmitted during each specified period. Namely,

[0074] between t=0 and t=T, R_(t)=R₀−R_(L),

[0075] between t=T and t=2T, R_(t)=R₀−2R_(L), and

[0076] between t=2T and t=3T, R_(t)=R₀−3R_(L)=R₁ are provided to themobile station as the new cell radius values to which a cell formationchange is proposed. If the mobile station is located at a distanceL_(B), the radius changes proposed during t=0 and t=2T do not affect themobile station, and no further action is necessary. However, the radiuschange proposed during t=2T and t=3T will make the mobile station becomeoutside the service area. Therefore, the mobile station transmits thechange stop signal to the base station such that the cell radius changethat makes the mobile station outside the area is stopped.

[0077] Next, details of the processing procedure of the secondembodiment are explained.

[0078]FIG. 8 and 9 are flowcharts showing the processing procedure ofthe base station, and FIG. 10 is a flowchart showing the processingprocedure of the mobile station.

[0079] First, the processing procedure of the base station is explained,referring to FIG. 8 and FIG. 9.

[0080] When a cell radius change is decided upon by the base station, aradius decrement interval R_(L)is obtained by dividing the differencebetween the new cell radius after change R1 and the present cell radiusR₀ by the number of times N of change as indicated by the followingformula (Formula 2) (S21).

R _(L)=(R ₀ −R ₁)/N  (Formula 2)

[0081] Next, a radius R_(t) that the base station changes to in steps isobtained by the following formula (Formula 3) (S22).

R _(t) =R ₀ −R _(L)  (Formula 3)

[0082] The above processes are performed by the cell formationprocessing unit 15. The resulting R_(t) is provided to the informationsignal processing unit 16. The information signal processing unit 16generates an information signal including data of R_(t) and the presenttransmission electric power PS, transmits the information signal to themobile station in the cell periodically and repeatedly through thetransmitting unit 17, operates a timer, and waits for responses from themobile stations (S23). The time length from the transmission start timeof the information signal to the scheduled implementation time of thecell radius change is set at T as shown in FIG. 7. After waiting for theresponse for the predetermined time, i.e., after a time out (YES atS24), the base station determines whether a change stop signal has beenreceived from one or more mobile stations (S25). If a change stop signalhas been received from one or more mobile stations (YES at S25),transmission of the contents of the proposed cell change is stopped, thevalue of the cell radius setting value set up in the cell formationprocessing unit 15 is deleted, and the cell radius change processing isstopped such that the mobile station that transmitted the change stopsignal can remain in the service area. That is, if the new cell radiusR₁ is set equal to R_(t) (S26), the cell radius is made the smallestpossible while keeping the mobile station within the service area.

[0083] On the other hand, if it is determined after the timeout (YES atS24) that no change stop signals have been received from any of themobile stations (NO at S25), the process reached the final stage and iscompleted if R_(t)=R₁ (YES at S27). If R_(t) is not equal to R₁ (NO atS27), R_(t) is set at R_(t)−R_(L), and provided to the transmitting unit(S28), and the process returns to the first step.

[0084] Next, the processing procedure of the mobile station is describedin reference to FIG. 10.

[0085] The information signal from the base station with whichcommunications are ongoing is received by the receiving unit 22 of themobile station, and provided to the information signal detection unit 25that extracts data such as the received electric power Pr, the cellradius Rt after change and the present transmission electric power PS ofthe base station included in the information signal (S31), and sends thedata out to the distance calculation unit 24. The distance calculationunit 24 calculates the distance Li between the mobile station and thebase station by applying the received electric power Pr and PS to theabove-mentioned Formula 1 (S32).

[0086] In addition, although the present embodiment employs Formula 1,for example, for calculating the distance between the mobile station andthe base station, another formula for the relationship between thesignal electric power and the distance suitable for the propagation pathcharacteristics of the system can be used, and the present invention isnot limited to using Formula 1.

[0087] When the distance calculation unit 24 has calculated L_(i) asmentioned above, the calculation result is provided to the cell radiuscomparison unit 26. The cell radius comparison unit 26 compares R_(t)with L_(i) (S33). If the comparison determines that R_(t) is larger thanL_(i) (NO at S33), which indicates that the mobile station concernedwill remain within the service area after the cell radius is changed,the process ends and returns to the first processing step.

[0088] Otherwise, if it is determined that R_(t) is smaller than L_(i)(YES at S33), the process moves to the next step, where it is determinedwhether a hand-over to another base stations is possible (S34). Thisdetermination (S34) is performed as follows.

[0089] The hand-over processing unit 27 receives the information itemssent one by one from the connectable base station detection unit 23, anddetermines the availability of a connectable base station for hand-overfrom the received information. The result of the determination (theinformation about the availability of a connectable base station that isacquired as a determination result in this case) is periodicallyprovided to the cell radius comparison unit 26. The cell radiuscomparison unit 26 determines whether the hand-over to another basestation is possible using the information about the availability of aconnectable base station provided by the hand-over processing unit 27.

[0090] The cell radius comparison unit 26 directs the control signalprocessing unit 28 to generate the control signal that is needed for themobile station to carry out the hand-over, when it is determined thatthe hand-over to another base station is possible by the above-mentionedcheck (S34) (NO at S34). Using the control signal, the mobile stationperforms the hand-over to another base station, and continuescommunications.

[0091] Otherwise, when it is determined that the hand-over to anotherbase station is impossible by the above-mentioned check (YES at S34),the mobile station concerned transmits a change stop signal to the basestation (S35). Thereby, the mobile station can stop cell radius changeprocessing of the base station, and can avoid disconnection of ongoingcommunications.

[0092] Next, an outline of the third embodiment of the cell radiuschange processing is described with reference to FIG. 11, wherein a basestation changes cell formation based on the cell formation controlmethod of the present invention.

[0093] As for FIG. 11, like FIG. 7, the horizontal axis expresses thetime lapse from the time when the base station decides to make a cellradius change, and the vertical axis expresses the distance (cellradius) from the base station concerned. Here, in the base station cell,R₀ represents the present cell radius, and R₁ represents the new cellradius after change. In the third embodiment of the present invention,the cell radius change from R₀ to R₁ is divided in N stages. Forexample, the cell radius change is carried out in three steps (N=3) inthis embodiment, namely, the cell radius is to be reduced from R₀ toR_(t)=R₀−R_(L), R_(t)=R₀−2R_(L), and then to R_(t)=R₀−3R_(L)=R₁, asshown in this figure. Specifically, the following R_(t) values of stagechange cell radius are transmitted during each specified period. Namely,

[0094] between t=0 and t=T, R_(t)=R₀−R_(L),

[0095] between t=T and t=2T, R_(t)=R₀−2R_(L), and,

[0096] between t=2T and t=3T, R_(t)=R₀−3R_(L)=R₁ are provided to themobile station as the new cell radius to which the cell formation isproposed to be changed.

[0097] In the third embodiment, information about the final cell radiusR₁ is added to the information signal in each stage, which is inaddition to the second embodiment.

[0098] In this example, if the distance between the mobile station andthe base station is L_(B), the cell radius change proposed during t=0−2Tis acceptable, and no special process is needed. However, in the thirdembodiment, the final cell radius R₁ that the base station willultimately propose is also provided from the beginning of t=0 to themobile station. Therefore, the mobile station can start attempting tofind another base station, a second base station, for hand-over at anearlier stage. Further, even when the second base station is in theprocess of changing its cell radius, the mobile station can determinewhether the final cell radius R₁ of the second base station will containthe mobile station. In this manner, the number of times of hand-over canbe decreased. Further, even when there is no candidate base station towhich the distance from the mobile station is shorter than the finalcell radius of the base station, it is possible to choose a base stationfor hand-over, with which communication can be maintained for thelongest possible time, based on R₁ and the stage change cell radiusR_(t). Consequently, since the number of times of hand-over can beminimized, and transmission and reception of control signals, such as achange stop signal, can be minimized, wireless resources can be usedefficiently.

[0099] Next, the detailed processing procedure of the third embodimentis explained.

[0100]FIG. 12 and FIG. 13 are flowcharts showing the processingprocedure of the base station, and FIG. 14 and FIG. 15 are flowchartsshowing the processing procedure of the mobile station.

[0101] First, the processing procedure of the base station is explained,referring to FIG. 12 and FIG. 13.

[0102] When the base station decides to make a cell radius change, aradius decrement interval R_(L) of the cell radius is obtained bydividing the difference between the present cell radius R₀ and the finalcell radius after change R₁ by the number of times of change N (refer toFormula 2) (S41). Next, the base station obtains R_(t) from thedifference between R₀ and R_(L), using Formula 3 (S42), R_(t) being thestage change cell radius that the base station uses when graduallychanging the cell radius. The processes are performed by the cellformation processing unit 15. The information signal processing unit 16generates information containing data such as R₁, R_(t), P_(s), R_(L),and T, and transmits the information to mobile stations in its cellperiodically and repeatedly through the transmitting unit 17, operates atimer, and waits for responses from the mobile stations (S43). The timelength from the time when transmission of the information signal isstarted to the scheduled time of cell radius change implementation isset at T as shown in FIG. 11. After waiting for the responses for thepredetermined time, that is, after a timeout (YES at S44), it isdetermined whether a change stop signal is received from one or moremobile stations (S45). If it is determined that a change stop signal isreceived from one or more mobile stations (YES at S45), transmission ofthe information of the proposed cell change is stopped, the value of thecell radius set in the cell formation processing unit 15 is deleted, andthe cell radius change processing is stopped, such that the mobilestation that transmitted the change stop signal can remain in theservice area. By replacing R₁ with R_(t) (S46) the cell radius is set atthe smallest possible while keeping the mobile station in questionwithin the service area.

[0103] Otherwise, if it is determined after the timeout (YES at S44)that no change stop signals are received from any of the mobile stations(NO at S45), the process reaches the final step and is completed ifR_(t)=R₁ (YES at S47). If R_(t) is not equal to R₁ (NO at S47), R_(t) isreplaced with R_(t)−R_(L), which is provided to the transmitting unit 17(S48), and the process returns to the first step.

[0104] Next, the processing procedure of the mobile station is explainedwith reference to FIG. 14 and FIG. 15.

[0105] The processing procedure of the mobile station in the thirdembodiment is similar to the processing procedure of the secondembodiment, as shown in this figure. However, the mobile station in thethird embodiment is capable of selecting a base station that cancontinue connection longer than others as the hand-over destination.That is, the mobile station can receive the information signal from acandidate base station, and calculate remaining time until the cellradius of the candidate base station reaches R₁, using R_(t), R₁, R_(L),and T contained in the information signal. In this manner, the mobilestation can attempt to hand-over to the candidate base station beforethe mobile station becomes outside the service area of the candidatebase station.

[0106] Further, the mobile station is capable of calculating thedistance to the candidate base station from P_(S) and the receivingsignal level of the information signal from the candidate base station.By comparing the calculation result with R₁, the mobile station candetermine whether the mobile station will remain within the service areaafter the cell radius change. Using this information, the mobile stationcan select a base station to communicate with such that the number oftimes of hand-over is minimized.

[0107] Although the embodiments described above are based on the basestation autonomously controlling the cell formation, the presentinvention is applicable to a structure wherein a central station thatgoverns a plurality of base stations controls the cell formation. Insuch a structure, the cell formation control is based on informationacquired from the base stations governed by the central station. Thecentral station may be, for example, a wireless circuit control station.

[0108] In the embodiments, the cell formation control function of thecell formation processing unit 15 of the base station includes a cellformation change stop means, a cell formation change means, and a cellradius gradual change means, and the information function of theinformation signal processing unit 16 includes a first cell informationproviding means and a second cell information providing means.

[0109] Further, the comparison function of the cell radius comparisonunit 26 of the mobile station represents a preparatory outside-the-celldetection means, the signal transmitting function of the transmittingunit 29 represents a detection result providing means and a distanceinformation providing means, the distance calculation function of thedistance calculation unit 24 represents a distance calculation means,and the hand-over processing function of the hand-over processing unit27 represents a first hand-over means and a second hand-over means.

[0110] As mentioned above, the cell formation control method and themobile communications system of the present invention can prevent adisconnection of communication that is due to a reduced cell radiusexcluding a mobile station in communication from occurring by the mobilestation being capable of informing the base station whether a plannedcell radius reduction is acceptable, and by the base station reducingthe cell radius enough to cover the mobile station based on theinformation from the mobile station.

[0111] The present invention further realizes a base station that canchange cell formation according to the above cell formation controlmethod.

[0112] The present invention further realizes a mobile station that canmaintain communication without being disconnected, according to theabove cell formation control method.

[0113] Further, the present invention is not limited to theseembodiments, but various variations and modifications may be madewithout departing from the scope of the present invention.

[0114] The present application is based on Japanese priority applicationNo. 2001-274214 filed on Sep. 10, 2001 with the Japanese Patent Office,the entire contents of which are hereby incorporated by reference.

What is claimed is
 1. A cell formation control method that controlsformation of a cell, which is performed by a base station while the basestation communicates with a mobile station in the cell, comprising: astep wherein the base station provides the mobile station withinformation relative to cell formation change, a step wherein the mobilestation determines whether the mobile station will be placed outside thecell if the cell formation change is performed, in advance of the basestation performing the cell formation change, based on the informationrelative to the cell formation change, a step wherein a result of thedetermination by the mobile station is provided to the base station, anda step wherein the base station suspends the cell formation change basedon the result of the determination provided by the mobile station.
 2. Acell formation control method as claimed in claim 1, further comprising:a step wherein the mobile station calculates a distance between themobile station and the base station, when the mobile station determinesthat the mobile station will be placed outside the cell if the cellformation change is performed, a step wherein the mobile stationprovides the base station with information relative to the distance, anda step wherein the base station performs the cell formation change basedon the information relative to the distance.
 3. A cell formation controlmethod as claimed in claim 1, further comprising a step wherein themobile station attempts a hand-over to another base station, when themobile station determines that the cell formation change will place themobile station outside the cell.
 4. The cell formation control method asclaimed in claim 1, further comprising a step wherein the base stationprovides at a predetermined interval within a predetermined period allthe mobile stations in the cell with the information relative to thecell formation change, in advance of the cell formation change beingperformed.
 5. A cell formation control method that controls formation ofa cell, which is performed by a base station while the base stationcommunicates with a mobile station in the cell, comprising: a stepwherein the base station provides the mobile station with informationrelative to cell formation change, where the cell formation change is tobe performed in two or more stages, including a final stage, at apredetermined interval, a step wherein the mobile station determineswhether the mobile station will be placed outside the cell if the cellformation change is performed through the final stage, and a stepwherein the mobile station attempts a hand-over to another base stationif the mobile station determines that the mobile station will be placedoutside the cell if the cell formation change is performed through thefinal stage, even if the mobile station will remain within the cell inany of prior stages.
 6. The cell formation control method as claimed inclaim 5, further comprising a step wherein the base station provides ata predetermined interval within a predetermined period all the mobilestations in the cell with the information relative to the cell formationchange, in advance of the cell formation change being performed.
 7. Thecell formation control method as claimed in claim 5, further comprisinga step wherein the base station provides all the mobile stations in thecell with the information relative to the cell formation change, inadvance of the cell formation change being performed, at a predeterminedinterval within a predetermined period, where the information relativeto the cell formation change contains information about a cell radius ofa next stage and a cell radius of the final stage.
 8. A mobilecommunications system configured by two or more mobile stations and abase station that controls formation of a cell, comprising: preparatoryoutside-the-cell detection means that enables each of the mobilestations to determine whether the mobile station will be placed outsidethe cell if a cell formation change is performed, in advance of the basestation performing the cell formation change, based on informationrelative to the cell formation change provided to the mobile station bythe base station, detection result providing means that enables themobile station to provide a result of the preparatory outside-the-celldetection to the base station, and cell formation change stop means thatenables the base station to suspend the cell formation change based onthe result of the determination provided by the mobile station.
 9. Themobile communications system as claimed in claim 8, further comprising:distance calculation means that enables the mobile station to calculatea distance between the mobile station and the base station, when themobile station determines that the mobile station will be outside thecell, distance information providing means that enables the mobilestation to provide the calculation result as distance information to thebase station, and cell formation change means that enables the basestation to perform the cell formation change based on the distanceinformation provided by the mobile station.
 10. The mobilecommunications system as claimed in claim 8, further comprising firsthand-over means that enables the mobile station to attempt acommunication connection change to another base station when the mobilestation determines that the mobile station will be placed outside thecell.
 11. The mobile communications system as claimed in claim 8,further comprising first cell information providing means that enablesthe base station to provide all the mobile stations in the cell with theinformation relative to the cell formation change at a predeterminedinterval within a predetermined period, in advance of the cell formationchange being performed.
 12. A mobile communications system configured bytwo or more mobile stations and a base station that controls formationof a cell, comprising: cell radius gradual change means that enables thebase station to perform the cell formation change in two or more stages,including a final stage, at a predetermined interval, and secondhand-over means that enables the mobile station to attempt acommunication connection change to another base station, when the mobilestation determines that the mobile station will be placed outside thecell if the cell formation change is performed through the final stage,even if the mobile station will remain within the cell in any of priorstages.
 13. The mobile communications system as claimed in claim 12,further comprising first cell information providing means that enablesthe base station to provide at a predetermined interval within apredetermined period all the mobile stations in the cell with theinformation relative to the cell formation change, in advance of thecell formation change being performed.
 14. The mobile communicationssystem as claimed in claim 13, further comprising second cellinformation providing means that enables the base station to provide ata predetermined interval within a predetermined period all the mobilestations in the cell with the information relative to the cell formationchange, in advance of the cell formation change being performed, wherethe information relative to the cell formation change containsinformation about a cell radius of a next stage and a cell radius of thefinal stage.
 15. A base station that changes cell formation according toa situation of adjacent cells, while communicating with a mobile stationin the cell, comprising cell formation change stop means that suspendsthe cell formation change based on information provided by the mobilestation.
 16. The base station as claimed in claim 15, further comprisingfirst cell information providing means that enables the base station toprovide information about a cell radius to which a change is scheduled,to all the mobile stations in the cell at a predetermined interval in apredetermined period before the change is performed.
 17. The basestation as claimed in claim 16, further comprising cell radius gradualchange means that enables the base station to change the cell radiusgradually in stages at a predetermined interval until it reaches a finalcell radius.
 18. The base station as claimed in claim 17, furthercomprising second cell information providing means that enables the basestation to provide the mobile station, at a predetermined intervalwithin a period, information about a cell radius corresponding to a nextstage, and the final cell radius, where the cell radius is graduallychanged at a predetermined interval.
 19. A base station that changescell formation according to a situation of adjacent cells, whilecommunicating with a mobile station in the cell, comprising cellformation change means that changes the cell formation according to aparameter that can be revised based on information from the mobilestation, such that the mobile station can remain within the cell afterthe cell formation change.
 20. A mobile station that communicates with abase station, comprising: preparatory outside-the-cell detection meansthat enables the mobile station to determine whether the mobile stationwill be placed outside a cell of the base station if the base stationperforms a cell formation change prior to the cell formation changebeing performed, and detection result providing means that enables themobile station to provide the result of the preparatory outside-the-celldetection to the base station.
 21. The mobile station as claimed inclaim 20, wherein the preparatory outside-the-cell detection meansfurther comprises: distance calculation means that calculates a distancebetween the mobile station and the base station when the mobile stationdetermines that the mobile station will be placed outside the cell, anddistance information providing means that provides the base station witha result of the calculation performed by the distance calculation means.22. The mobile station as claimed in claim 20, further comprising firsthand-over means that enables the mobile station to attempt acommunication connection change to another base station, when the mobilestation determines that the mobile station will be placed outside thecell as the result of the preparatory outside-the-cell detection.
 23. Amobile station in a cell, which communicates with a base station,comprising second hand-over means that enables the mobile station toattempt a communication connection change to another base station, whenthe mobile station determines from information relative to a cellformation change provided to the mobile station by the base station,that the mobile station will be placed outside the cell if the basestation performs the cell formation change through a final stage, evenif the mobile station will remain within the cell in any of priorstages.